Codecs raise RuntimeError at unexpected end of file in Python 3.7+

[goodmami]

Since Python 3.7, a StopIteration error raised inside a generator is converted into a RuntimeError at the point that it leaves the generator, as detailed in PEP-479. The delphin.util.LookaheadIterator wraps an iterator and suppresses the first StopIteration in order to provide the L1 lookahead. This will need to be fixed somehow. Hopefully it's just a matter of using return instead of raise, but otherwise it would require a custom error class or something.